Again, you are questioning reality with your imagined scenario. If it was indeed profitable, it would be in mass usage by now. The REALITY is that it is NOT profitable to run a ASIC as a heater, because it CAN NOT be turned on all the time, therefore the hardware cost can not be easily recouped. It has no hope to compete against a full time mining farm.
Some heaters are on all the time, but lets assume that we have intermittent Asics competing with full time Asics for a moment.
Option A-
ASIC hot water heater running 20% of the time cost efficiency = (amortized costs of design, testing, build, and shipping / millions of units) - 20% normal rate of transaction fees and block rewards compared to standalone miners
Option B-
ASIC Standalone miner running 100% of the time cost efficiency = (amortized costs of design, testing, build, and shipping / thousands of units) + electricity costs + possible costs for cooling
Do the math . What is more efficient?
P.S... remember in option A electricity isn't included because its a needed product already consumed.